Technical White Papers

As any veteran troubleshooter knows, often the biggest challenge in diagnosing a process performance problem is backtracking from symptom to cause. Sometimes the path leading from problem to cause is short and easily followed, such as when a motor fails and needs to be replaced. But when the trail is lengthy, indirect or unclear, the possibilities of cause and effect quickly multiply, often branching into a seemingly endless array of frustrating dead-ends. When faced with an apparent loss-inweight feeding problem the feeder itself rightly becomes the initial focus of troubleshooting scrutiny. But what if the feeder checks out yet the problem persists? If the feeder has passed muster, the problem's underlying cause must then lie elsewhere, whether with the operating environment, upstream/downstream conditions or equipment, or with the process material itself.

While it may be easy to obtain information on the acquisition cost of a new feeder line, it is more difficult to evaluate less obvious costs and potential savings of ongoing operation. Too often the savings an efficient feeding system can produce over time are not considered at the time of purchase and only the "up front" capital cost is reviewed. Like any process equipment, a feeding system costs something to acquire, performs a function, and costs something to support and sustain its ability to function as desired. Together, these three factors constitute the Total Cost of Feeding.

In the mid-1970's, Coperion K-Tron revolutionized bulk solids feeding with its introduction of the first truly digital load cell specially designed for process weighing applications. Based on an innovative vibrating wire concept, the new digital technology soon proved to be a significant advance over the analog LVDTs and strain gauges then in widespread use. This technical paper provides a detailed history about the vibrating wire theory, including the Smart Force Transducer (SFT) product evolution from the Digital Mass Transducer MK-II to today's Smart Force Transducer II & III . Article also explains the SFT's application to dry bulk material feeders and meters, including loss-in-weight feeders, weigh belt feeders, and the gravimetric flow meter.

Loss-in-weight feeders have evolved from mechanically ponderous devices to the sophisticated microprocessor controlled instruments of today. Weighing and control advancements over the years have made loss-in-weight (gravimetric) feeding the preferred method wherever the combination of high gravimetric accuracy, ingredient containment, and material handling capability are needed.

However, loss-in-weight feeding does possess some shortcomings, especially at higher feed rates. First, during the feeder's required hopper refill phase, weight-based control must be temporarily suspended and replaced with volumetric control. It is in this refill phase that significant feed rate errors can occur due to volumetric-control inaccuracies. And second, higher feed rates have historically meant physically large and expensive systems. In some cases required space could only be obtained at the cost of significant structural changes to the plant itself.

Simply operating within established ingredient tolerances may not be good enough anymore. A new, practical and application-specific measure of blend uniformity underscores the importance of feeding accuracy and suggests an effective strategy to achieve improved formulation consistency. Common to all processors is the concept of a recipe. Wherever two or more materials are combined, a recipe exists. Whether it’s called a recipe, a formulation, blend, or compound, the notion is the same: to specify ingredient proportions to create a product possessing certain carefully defined properties, attributes or characteristics. Small-scale processing operations may proportion ingredients manually (either by volume or weight), but continuous or batch feeders usually perform the proportioning operation where throughputs are higher.

Bins may perform a simple function, but their design is crucial to keeping process material on the move. Anyone who has had to resort to sledgehammer blows to persuade materials to flow from a bin knows of the complexities and difficulties of bin design. Before the physics of material storage and flow was even marginally understood, such a heavy-handed approach mainly served to characterize the frustration in solving the apparently simple problem of making material move from a container with a hole in the bottom. Simple problems don’t always have simple solutions, though. Getting material to obey the law of gravity can be a perplexing case in point.